When the rice is cooked, let rest for 10 minutes. Pour the rice in a large glass or wood bowl. Mix in the sushi seasoning. Set aside and slightly covered. Let the rice cool at room temperature for 30 minutes or so.
In a small bowl, combine the mayonnaise and Sriracha sauce. Set aside.
For the tuna, pat it dry with paper towels. First, cut the tuna against the grain into ¼-inch slices. Then, cut the tuna slices into ¼-inch cubes. Place the tuna in a medium bowl and leave in the fridge until ready to use.
To assembly the rice bowl, put desired amount of sushi rice into a bowl. Top with tuna cubes, spicy tuna sauce, cucumber, avocado, green onions and sesame seeds. Serve immediately.